The long term objective of this project is to gain insight into the regulation of optic nerve head (ONH) blood flow with varying neural activity, metabolic demand and vasodilator release. This will be achieved with our unique experimental system for quantifying ONH blood flow by laser Doppler flowmetry (LDF) combined with localized chemical measurements with excellent spatial and temporal resolution. Single or double barrel electrochemical microsensors (tips <5 microm) sensitive to nitric oxide (NO), tissue oxygen tension (PO2) or potassium ion (K+) will be used to make measurements in ONH tissue of anesthetized cats. These data will be used to test hypotheses that (1) ONH blood flow, neural activity and 02 metabolism are coupled, (2) NO plays an important role in regulating ONH blood flow, and (3) that the cyclo-oxygenase pathway modulates 02 and NO control of ONH blood flow. The relative importance that local NO, PO2, and K+ play in regulating ONH blood flow will be determined with flickering light stimuli at different frequencies and luminance levels, during increased intraocular pressure, during hypoxia and hypercapnia and following ischemia by temporarily occluding the ophthalmic artery. Regional increases in 02 metabolism with flicker will be quantified from changes in ONH blood flow and PO2. Coupling of ONH blood flow with NO will be assessed by blocking NO synthase. Coupling between ONH blood flow and the cyclo--oxygenase pathway will be determined by blocking with indomethacin. Spatial variations in relationships between ONH blood flow, chemical microenvironment and O2 metabolism will be characterized. ONH blood flow regulation and local chemical responses will be measured for control conditions, after blocking NO synthase, after blocking cyclo-oxygenase, and with both NO synthase and cyclo-oxygenase blocked. These studies are important for understanding how normal ONH blood flow is regulated, and may provide insight into impaired mechanisms with pathological conditions such as glaucoma or neurodegenerative diseases.